1. Field of the Invention
The present invention relates to a water injection amount control system for a fuel and water injection engine applied to the engine arranged to inject two fluids of fuel and water to a combustion chamber.
2. Description of Related Art
Hitherto, various technologies for reducing the discharge of NOx by lowering combustion temperature by injecting water together with fuel into a combustion chamber of an engine have been proposed. For example, Japanese Unexamined Patent Publication No. Hei. 8-226360 has disclosed a fuel and water stratified injection system for injecting in the order of fuel, water, and fuel in stratification from one and same injection nozzle.
According to such fuel and water stratified injection system, water is supplied to one and same injection nozzle in advance during the interval period between injections so that fuel, water and fuel are geometrically stratified in this order and the water and fuel are injected to a cylinder in stratification in this order in one time of injection. It then allows flame temperature to be lowered and the emission of NOx, PM (particulate matter) and others to be reduced.
FIG. 15 is a schematic diagram showing the structure of an engine (fuel and water injection engine) equipped with the fuel and water stratified injection system which has been proposed since the past.
As shown in FIG. 15, the engine comprises a main body 100 of the engine, a fuel injection pump 101, a water supply pump 102, an ECU (controller) 103, an intake passage 104 and an exhaust passage 105. The ECU 103 decides the rack position Rw2 of the water supply pump 102 based on engine speed Ne obtained from an operating speed of the fuel injection pump 101 and the rack position Rw1 of the fuel injection pump 101.
Meanwhile, beside those described above, an exhaust gas recirculating system (EGR system) has been well known as means for reducing NOx and has been already put into practical use. The EGR system slacks combustion within a cylinder by recirculating part of exhaust gas of an engine to an induction system to lower combustion temperature and to reduce NOx within the exhaust gas of the engine.
Then, Japanese Unexamined Patent Publication No. Hei. 9-144606 has disclosed a technology for reducing NOx without increasing HC and smoke by combining the fuel and water stratified injection system with the EGR system.
According to this technology, NOx is reduced by actuating only the EGR system when the engine load is below a preset value and NOx is reduced by injecting water in addition to the EGR system when the engine load is in the range exceeding the preset value.
By the way, because the more water is injected, the more the combustion degrades within the cylinder, flameout occurs when water is injected too much. Then, the maximum water injection amount has been set so as not to inject water more than this maximum water injection amount before reaching to the limit of causing the flameout in the past. The maximum water injection amount has been defined based only on engine speed and engine load.
Meanwhile, as a result of the further study on the relationship between the EGR system and the injection of water, the inventors et al. of the present application have obtained a finding that the limit of flameout caused by the injection of water rises when the EGR rate (or EGR amount) increases. Accordingly, it is conceivable that the water injection amount and the maximum water injection amount may be raised corresponding to the increase of the EGR rate and that the effect for reducing NOx may be enhanced further by setting as such.
However, because the related art technology described above has defined the maximum water injection amount only by the engine speed and the engine load, it has been unable to set the water injection amount corresponding to the EGR rate and the NOx reducing effect has been limited naturally.